The spray works against spider mites as long as the nutrient tea is not added, but the thing that worked best for me was insecticidal soap mixed with neem oil. The soap suffocates the adult mites and the neem kills the eggs by permeating the thin membrane that separates them from the outside world. I had a very bad spider mite infestation in a room a few years ago, I sprayed every day for 7 days, then every other day for 7 days, then every 3 days for the rest of the month. After that I never saw another spider mite in my room. It was my fault though for buying a used light and not cleaning it properly.
Regarding consumption of leaves fruits, do not spray for the last week before harvest. For continual harvesting plants just take care to not spray the sections you will be harvesting in the next week. I harvest jalapeno and habanero peppers once a week and never had a problem with any leftover residue, just wash thoroughly as usual.
The reason I use lemon grass is we have no insects here that will eat it, not even ants. That and the locals have been using it forever:)